The term ‘axis’ is used here in the sense of the motor axis and the moving motor parts as well as all the parts that are driven by it, such as the gearbox, etc. The term ‘textile machine’ is used in the sense of any facility for the production, processing, treatment, handling or transporting of textile material and products made from textile material, such as a weaving machine, a tufting machine, a knitting machine, a bobbin winder, a winding machine, etc.
A textile machine generally comprises several independently operating motors that are controlled to set respective interacting parts of the textile machine in synchronized motion. These controls can be position-synchronous, angle-synchronous or speed-synchronous or any combination of these, whereby the synchronization of the axes can have both a fixed ratio (‘electronic gearing’) and a continuously varying ratio (‘electronic camming’).
A condition without power supply can, for example, be the result of an unexpected failure of or a reduction in the mains voltage due to an interruption or a fault in the power supply or due to a fault in the textile machine itself or due to an automatic shutdown for safety reasons, but can for example also be the result of a deliberate switching-off of the machine or the power supply.
When the power supply to such a machine fails, it is important in all cases that the various parts of the machine remain in synchronous motion until they have come to a standstill. This avoids collisions between different parts of the machine (e.g. between the reed and the cutting knife of a face-to-face weaving machine) or between a machine part and the textile material, such as yarns (e.g. between a rapier and the warp threads on a weaving machine), so preventing damage to the machine and/or the textile material. Furthermore, the different parts of the machine come to a standstill in such relative positions that the textile machine is immediately ready to start again without any additional recovery procedures.
The international patent publication WO 97/02650 describes a method in which in the event of a failure of the power supply, an energy-dominant axis (this is the axis with the highest kinetic energy) is selected and that this axis is gradually slowed down while the released energy is used to supply the logic units of the different drives via a common intermediate circuit (hereinafter also referred to as ‘common DC bus’). Taking into account the known inertia of the load coupled to the various axes, the angular velocity and the current consumption by these axes, an optimum, ideal speed curve during the run-down to come to a standstill of the textile machine is calculated for the whole system.
In order to limit transitional phenomena between the moment of the mains failure and the following of this ideal speed curve, a specific initial speed curve is first calculated for each individual axis on the basis of the values of the same parameters of each axis at the moment of the mains failure. The controller regulates the speed of each individual axis according to the initial speed curve calculated for that axis. Subsequently one controller takes over the speed control of the whole system, whereby all the axes come to a standstill with synchronous speed and fixed synchronization ratio and with an evolving speed according to the calculated speed curve, while the voltage on the common DC bus is held constantly at a constant value.
This method is typically applicable to machines of which the inertia and the load on the different axes are predictable, and the different axes can be brought to a controlled standstill via individual motor controllers.
This method has the disadvantage, however, that it is not applicable to systems such as weaving machines where such a theoretical and initial curve of the speed decrease cannot be calculated for various reasons, such as inter alia because the inertia varies as a function of the angular position of the part, because the load varies as a function of the angular position of the part as a consequence of non-linear mechanisms, because the load is not known in advance, because the loads change strongly as a function of time, such as evolving frictional losses, or because the different parts do not rotate in a speed-synchronous manner, but in a position-synchronous manner relative to one another with continuously changing ratios.
Energy recuperation by defining in advance the speed profile to be followed, and hence the speed variation, as described in WO 97/02650 also imposes dynamic limits on the components connected to a common DC supply. Since an axis with a high energy content typically allows energy recuperation, whereby this axis generally also has a higher inertia than the components connected via the intermediate circuit that have to be supplied in the event of a mains fault or mains failure, the energy change in the event of a mains fault or mains failure will be slower than for the other connected components to be supplied.
This means that the extent to which energy is recuperated is completely dependent on the mechanical properties of this axis, namely inertia and friction. The dynamic behaviour of energy recuperation by varying the speed of a primary axis is not sufficient if the other axes connected to the common DC bus react highly dynamically (positioning adjustments), in other words if the bandwidth of the positioning control loops is far higher than the bandwidth of the speed controller of the regenerating axis.
A further disadvantage with this manner of energy recuperation is that due to the continuously decreasing speed, the energy gain varies continuously with the same change in speed. At low machine speeds, this can result in unstable control of the voltage on the intermediate circuit, and this is certainly the case if the loads connected to the intermediate circuit are of the highly dynamic kind.